Golang: Mirror all writes to stdout and stderr in program to log file

tee.go

package main

import (
	"fmt"
	"io"
	"log"
	"os"
)

func main() {
	fn := logOutput()
	defer fn()
	var words = "Hello\nDoctor\nName\nContinue\nYesterday\nTomorrow"
	for i:=0 ; i < 10 ; i++ {
		log.Println(i)
		fmt.Println(i)		
		fmt.Println(words)
	}
}

func logOutput() func() {
	logfile := `logfile`
	// open file read/write | create if not exist | clear file at open if exists
	f, _ := os.OpenFile(logfile, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0666)

	// save existing stdout | MultiWriter writes to saved stdout and file
	out := os.Stdout
	mw := io.MultiWriter(out, f)

	// get pipe reader and writer | writes to pipe writer come out pipe reader
	r, w, _ := os.Pipe()

	// replace stdout,stderr with pipe writer | all writes to stdout, stderr will go through pipe instead (fmt.print, log)
	os.Stdout = w
	os.Stderr = w
	
	// writes with log.Print should also write to mw
	log.SetOutput(mw)

	//create channel to control exit | will block until all copies are finished
	exit := make(chan bool)


	go func() {
		// copy all reads from pipe to multiwriter, which writes to stdout and file
		_,_ = io.Copy(mw, r)
		// when r or w is closed copy will finish and true will be sent to channel
		exit <- true
	}()

	// function to be deferred in main until program exits
	return func() {
		// close writer then block on exit channel | this will let mw finish writing before the program exits
		_ = w.Close()
		<-exit
		// close file after all writes have finished
		_ = f.Close()
	}

}

Extremely helpful!

// save existing stdout | MultiWriter writes to saved stdout and file
out := os.Stdout
mw := io.MultiWriter(out, f)

why not use stdout directly instead of making a copy?

thanks a lot, man.

This could definitely be used by a malicious actor.

1. Be super secret spy people
2. Make an open source library that lots of people use
3. Wait for your enemies to use your library in their super secret software.
4. Add code like this in an init function to your library and copy all of the output from your enemies' applications to your servers.